| Summary: | Cold subarctic pond ecosystems will be threatened due to the increase in global temperatures. Therefore, it is important to gain more knowledge on how their biota may respond to global warming. The aim of this research was to illustrate the variability in diatom species richness and community composition along environmental gradients in northernmost Finland and Norway and reveal the variables most strongly associated with diatom biodiversity. We also compared diatom biodiversity among different biotope types and placed emphasis on the distribution of rare diatom taxa. A total of 100 subarctic ponds from the Finnish and Norwegian Lapland covering an elevational gradient of 8-887 m above sea level were sampled. A generalised linear model and hierarchical partitioning were used to identify variables associated with diatom species richness. To identify variables associated with diatom community composition, a non-metric multidimensional scaling ordination was performed. Finally, a principal component analysis and permutational multivariate analysis of variance were used for the investigation of environmental and biotic differences among biotope types. Water pH, aluminium concentration, and air temperature best explained the variation in species richness and community composition. Diatom species richness and community composition did not differ among the biotope types, but environmental variables in pine forests differed significantly from other biotope types. Many diatom species occupied the entire elevational gradient, while rare taxa seemed to appear at the ends of the elevational gradient as well around the mid-elevational zone. We found that elevation was not sufficient to explain the variation in diatom species richness and community composition, but diatom biodiversity was shaped by a variety of local-scale environmental variables, some of which are in turn correlated with elevation. Our findings suggest that local abiotic factors and especially water chemistry are important factors in describing the ...
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